PROJECT SUMMARY Infections can cause irreversible damage to the brain. In this proposal, we aim to understand how the immune system is mobilized to specific regions of the brain where pathogens can be eliminated. The parasite Toxoplasma gondii causes a chronic brain infection that is kept under control by the immune system. Within the brain, the parasite persists in an intracellular cyst that appears to go unrecognized by the immune system. When cysts reactivate, vigorous immune responses occur in these regions. We find that cells of the myeloid lineage, which includes the brain-resident microglia and monocytes/macrophages that enter the brain from the blood, are densely packed in regions of cyst reactivation. We hypothesize that local microglia responses and the recruitment of immune cells to these sites is critical for the control of the parasite. In preliminary experiments, we infected mice where the majority of myeloid cells are unable to respond to IFN-g, the key cytokine required to control T. gondii. The mice were highly susceptible to infection and unable to control the parasite specifically in the brain, which demonstrates the importance of microglia and/or macrophages in this tissue. To determine if microglia were the key myeloid cell type responding to IFN-g, we generated mice with STAT1-deficient microglia. We find that these mice are more resistant to infection than mice with deficient macrophages and microglia, which implicates both cell types in the control of T. gondii in the CNS. Thus, we also began examining the factors that recruit macrophages to the brain and regions where parasite cysts reactivate. In preliminary studies, we find that two host molecules that are releasing from damaged and dying cells, IL-33 and IL-1a, are necessary to bring myeloid cells into the brain. Thus, in this proposal, we will address how: 1) microglia contribute to the control of T. gondii in the CNS through STAT1 2) the alarmin IL-33 drives chemokine production and protective myeloid cell response in the brain, and 3) IL-1a affects the vasculature to promote the recruitment of monocytes to the CNS. Together, addressing these aims will identify specific mechanisms by which myeloid cells limit parasite replication and protect the brain. The results from these studies may lead to the development of therapeutics that promote or prevent focal immune responses in the brain that may be beneficial for treating infections and brain tumors or for limiting unwanted inflammation in the brain.